A wide band forward coupler with balanced composite right-/left-handed transmission lines

This paper presents a novel wide band forward coupler with balanced composite right/left-handed (CRLH) transmission lines. The structures of conventional couplers using CRLH transmission lines have not realized the balanced CRLH transmission lines simultaneously in even/odd modes. Therefore, a coupled band of conventional forward couplers with CRLH transmission lines had been narrow. In the proposed couplers, stubs of the parallel CRLH transmission lines are connected each other and a part of the parallel CRLH transmission lines are coupled. By these structures, the proposed forward couplers are a balanced CRLH transmission lines simultaneously in even/odd modes. There are not the band gaps in the proposed coupler, thus, the proposed forward coupler has a wide band characteristics. The proposed forward coupler is simulated and experimentally demonstrated in the S-band. We have designed a 0dB forward coupler, and the return loss is lower than -10 dB in the range from 3.0 GHz to 4.3 GHz, which corresponds to a bandwidth of 36.5%, in the experiments.